device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permiss...

The routine gets called for every path component during lookup.
->i_mode is going to be cached on account of permission checks, while
->i_rdev is an area which is most likely cache-cold.

gcc 14.2 is kind enough to emit one branch:
	movzwl (%rbx),%eax
	mov    %eax,%edx
	and    $0xb000,%dx
	cmp    $0x2000,%dx
	je     11bc <inode_permission+0xec>

This patch is lazy in that I don't know if the ->i_rdev branch makes
any sense with the newly added mode check upfront. I am not changing any
semantics here though.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/20250416221626.2710239-3-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h
index d02f32b7514e647c7a16efa3cbf01e35b9e8066e..0864773a57e8fe6cf52396fcf5640e33f6de7a97 100644 (file)
--- a/include/linux/device_cgroup.h
+++ b/include/linux/device_cgroup.h
@@ -18,15 +18,16 @@ static inline int devcgroup_inode_permission(struct inode *inode, int mask)
 {
        short type, access = 0;
 
+       if (likely(!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode)))
+               return 0;
+
        if (likely(!inode->i_rdev))
                return 0;
 
        if (S_ISBLK(inode->i_mode))
                type = DEVCG_DEV_BLOCK;
-       else if (S_ISCHR(inode->i_mode))
+       else /* S_ISCHR by the test above */
                type = DEVCG_DEV_CHAR;
-       else
-               return 0;
 
        if (mask & MAY_WRITE)
                access |= DEVCG_ACC_WRITE;